1. Field of the Invention
The present invention relates to conductor circuits formed by electroless plating such as printed circuit boards, module circuit boards and LSI and a method for formation thereof.
2. Description of Prior Art
Hitherto, in a method for forming conductor circuits by depositing a metal at the bottom of grooves or holes formed in an insulator, namely, having a recessed section and filling the metal in the grooves or holes by electroless plating, control of the thickness of the built-up plating film has been conducted in the following manner. As controls, a plurality of test pieces having grooves or holes of the same structure as of the desired circuit board are introduced into a plating bath together with the desired circuit board. These test pieces are successively taken out with lapse of time and thickness of the plating film formed on the test pieces is calculated by electrolytic process or by measurement of weight. This has been conducted based on the assumption that the film thickness formed on the test piece and that on the desired circuit board are formed at the same speed.
However, according to the above method, errors necessarily occur because the circuit board and the test piece are not electrically connected, and especially, it is very difficult to determine the end point of the plating. Therefore, it is necessary that a plating thicker than desired is applied and the plated conductor portion rising from protrusions of the grooves or holes is abraded to make the plated surface of that portion to have the same level as the surface of the insulator. That is, an abrasion step has been necessary.
Furthermore, it has been impossible to simultaneously form conductor metal circuits differing in thickness by electroless plating on the same substrate. This is because when circuits are formed on the same substrate, it is impossible to control the plating speed and the plating time for each of the circuits having different thickness. At present, only the method of forming conductor circuits differing in thickness on the same substrate by electroless plating is to carry out the plating separately.
That is, a plating condition is set which is suitable for plating of the thinnest conductor circuit, and the first plating is carried out and then, the substrate is taken out from the electroless plating bath and the portion which has reached the desired thickness is masked with a plating resist. Thereafter, again a plating condition is set which is suitable for plating of the next thinner conductor circuit and the second plating is carried out. This operation must be repeatedly carried out as many as the times corresponding to the number of the portions differing in thickness of conductor circuits. Therefore, the above method of forming conductor circuits differing in thickness on the same substrate is very troublesome and has never been practically employed.
J. Electrochem. Soc., 140(8), 2229 (1993) discloses a method of electroless nickel plating with a plating solution in which an inhibitor for the cathodic partial reaction is dissolved. In this method, the nickel film formed on a flat substrate surface is prevented from producing a sharp difference in level from the surface of the substrate by adding an inhibitor for the cathodic partial reaction and a conical nickel deposit 5 of very gentle shape as diagrammatically shown in FIG. 6 is formed.
Furthermore, in the case of a conductor circuit in which recessions such as grooves or holes on the surface of the insulator are filled by electroless plating, as mentioned above, the surface is flattened by abrasion and as a result, the surface of the conductor circuit is poisoned at the time of abrasion or is oxidized when left in air. Therefore, when another conductor circuit is formed on such conductor circuit, the surface of the conductor circuit must be cleaned by etching or the like. However, it is difficult to control etching quantity at constant and if the etching is insufficient, the poisoned layer or oxide layer remains on the surface of the under conductor circuit to lower the adhesion to the upper conductor circuit to cause increase of contact resistance. If the etching is excessive, this causes formation of uneven surface of the upper conductor circuit or insufficient film thickness of the upper conductor circuit.